1. Field of Invention
The present invention is related to the field of electric power production and consumption. The present invention is further related to field of power electronics and power conversion systems involving AC frequency change.
2. Background--Prior Art
Methods for the interconversion of electrical power at a given input voltage, current, and frequency, to electrical power at a different output voltage, current, and frequency are well known in the art. The simplest device for such interconversion is the common transformer, which may be used to trade voltage for current with little loss of power to inefficiency. A transformer is not capable of altering the frequency of the power delivered to the load, and is not capable of functioning with DC power input.
More complex systems for power interconversion are capable of changing frequency and dealing with DC power input. The earliest such devices were motor/generator pairs and synchronous converters. These devices converted the input electrical power into an intermediate mechanical form, and then converted the mechanical power back into electricity. An AC induction motor, for example would act as the prime mover for a DC generator, thus allowing AC mains power to be converted to DC power. Similar devices may be used for frequency conversion.
Advances in semiconductor technology have made the field of power electronics viable for applications over a wide range of power levels. Power electronics is the application of electronic switching devices such as transistors to problems of power interconversion. Efficiency is of paramount importance in power electronic applications, in contrast to signal level electronics, in which fidelity of signal reproduction is of greater import.
The basic element found in power electronics applications is the switching element. An ideal switching element is either on, meaning that it has zero resistance, or off, meaning that it passes zero current. The transition between on and off in the ideal element is instantaneous. An ideal switching element would dissipate no power, either passing current without loss, or preventing the flow of current entirely. Again, this may be contrasted to signal electronics applications wherein active devices are operated in the linear mode, and therefore must dissipate power. Currently available semiconductor devices for the switching element approach ideal capabilities very well; a single transistor, which because of a slight imperfection must dissipate 10 watts of power as heat, may be capable of controlling several hundred watts of power delivered to a load.
Power dissipation in the switching elements may be divided up into several processes. Conduction loss is loss that occurs when the device is on. Conduction loss is similar to resistance loss; when current flows through a bulk material there is a slight voltage drop, which is accompanied by heat generation. Drive loss is the electrical power required to control the switching element. In something like a bipolar transistor, drive loss may be considerable. Switching losses are losses that result from current being carried during the transitional state between on and off or off and on. During this transitional state, the switch appears to be a high resistance, but considerable current may be carried. Often, switching losses, in particular, turn-off losses, account for the majority of the total losses of an operating device. Simply operating devices more slowly can substantially increase efficiency.
The simplest of the power electronics power converters to understand is the pulse width modulated DC controller, or chopper. This device is simply a switch element placed in series with a load and connected to a source of DC power. The switch element is switched on and off at a rapid rate. By varying the duty cycle of the switching element, i.e., by changing the ratio between `on` state and `off` state, the power delivered to the load may be varied. The variable duty cycle switching element acts as a variable resistance, without dissipating power in the fashion of an actual resistor. Though use of suitable filtering components such as inductors and capacitors, the pulsing nature of the DC power delivered to the load may be eliminated, and a smooth variable DC voltage may be delivered to the load. Additionally, such filter components allow one to trade current for voltage, and allow for output voltages greater than the input voltage. The latter device is known as a switch mode power supply or DC to DC converter. For many power applications, such as resistance heaters or lighting applications, pulsing DC is acceptable and the controller is simple in the extreme.
Of similar complexity is the SCR controller. The Silicon Controlled Rectifier, or SCR, is one of the oldest power electronic components, the semiconductor analog of the thryatron gas discharge tube. SCR devices are available with current capacities in the thousands of amperes and voltage ratings in the thousands of volts, meaning that a single device can switch many megawatts of power. The primary difficulty involved in the use of the SCR is that it is not self-commutating. Once an SCR is turned on by the application of a control pulse, it does not turn off. In order to commutate an SCR, the current flow must be stopped externally. Once current has ceased to flow, the SCR will `turn-off`, and will prevent current flow until the application of the next control pulse.
SCR devices may be used to great advantage when controlling AC current flows, wherein the flow of current necessarily stops and reverses twice with each AC cycle. The natural cessation of current may be used to commutate the SCR switch.
An SCR device may be used to control the power delivered to a load as follows: the SCR is connected in series with the load to a source of AC power. It is controlled by a device which is capable of triggering the SCR at a variable time after the beginning of the AC cycle; such devices are commonly called `cosine firing circuits`. If the SCR is triggered at the beginning of the AC cycle, then the SCR conducts for the entire half-cycle until the current flow reverses. The full power of the first half-cycle is delivered to the load. If the SCR is triggered near the end of the first half-cycle, then very little power is delivered to the load. The addition of a second SCR allows the second (negative going) half-cycle of the AC waveform to be similarly used. Thus through the use of two SCR devices and a simple delay circuit, control of the AC power delivered to a load is achieved. Packaged devices operating on this principal are used in home lighting dimmers and small motor controls.
Both the DC chopper and the AC SCR based controller may be used to produce an AC power output. Such output may be of use in supplying power to a conventional power distribution system, or for the operation of a motor requiring AC power input. Roughly, the methods described above are used, but the control circuitry is additionally used to vary the output in the same fashion that the desired AC output varies. In order to produce the desired output with reasonable fidelity, the pulse rate of the switching device must be of a higher frequency than the desired output frequency. A device which converts DC input power into an AC output in this fashion is commonly called an inverter.
Well known in the art is a device which consists of a rectification section for the production of DC power from input AC power, combined with a three phase inverter system for the production of AC. This AC power is then used to operate an AC motor. The benefit of a so called DC link converter is that the output AC power is of arbitrary and variable frequency and voltage, thus providing substantial control of motor operations. Such devices allow for control of motor synchronous speed, and with appropriate feedback devices can control the motor based upon speed, torque, acceleration, or other factors.
Also well known in the art is the cycloconverter. This is an SCR based converter for producing a polyphase AC output from a polyphase AC input. Commonly, this is a three phase device.
Background--Fidelity of AC Power Output
As stated above, the synthesized AC output from either the DC to AC inverter or the cycloconverter is composed of pulses, which approximate the desired AC output. The closeness of this approximation is enhanced by a greater number of pulses per AC output cycle. The error introduced by the pulsed nature of the inverter output exhibits itself in the form of harmonic distortion of the output waveform. Such harmonics, or high frequency components of the output, can degrade motor operation, cause overheating in power line transformers, cause excessive ground currents to flow, and lead to interference of other services. As the pulse rate is increased, the frequency of the harmonic distortion increases. The effect upon motors is reduced, and the ease with which the harmonics can be filtered is increased. Increasing pulse rate to enhance fidelity of the synthesized waveform thus has many advantages well known in the art.
Background--High Phase Order Machinery
Conventional AC electrical rotating machinery is either three phase or single phase in nature. Single phase systems are used for small applications, such as blowers, small power tools, and the like. Three phase systems are invariably used for applications of power ratings above about 5 horsepower. Archaic systems dating from the earliest polyphase electrification may use two phase machinery. The three phase power distribution system is a standard with many practical advantages.
However, the term encountered in the art for describing the three phase devices is `polyphase electrical rotating machines`. Furthermore, study of the mathematical equations which describe such machines shows that any number of phases may be used, and that the standard equations used for the design of electrical rotating machinery may be used for phase counts other than three. The substantial benefit to be had with the use of high phase order machines has been unrecognized until recently. `High phase order` is a term used in the art to describe alternating current of four or more phases as differentiated from the term `polyphase` which is normally used in the art to describe current of two or three phases despite its literal meaning of many phases.
High phase order machines are quite unknown in the industry, and have been the subject of comparatively few academic papers. High phase order induction machines are not present as specific embodiments in the US patent database, with high phase order brushless DC motors being only recently disclosed.
The only essential difference between a high phase order machine and a conventional three phase machine is that the number of independent phase windings in the stator (or as may be appropriate, rotor) is substantially greater than three. High phase order electrical rotating machinery makes use of similar laminations and frames, uses similar production techniques, similar inverter technology, similar materials, and provides similar services to those of three phase machines. Despite these similarities, the HPO design has numerous advantages over three phase machines.
1) Winding distribution factor is nearly unity. In conventional three phase machines, each phase must be applied to several windings which subtend a quite large region of the stator. The magnetic field produced by the coil at one side of this region is somewhat canceled by that of the coil at the opposite side of the region, thus reducing the effectiveness of the coils, and lowering the impedance. As a result, more turns of wire are needed to properly limit current flow and flux density. This larger number of turns means that the current must flow through a longer wire, while at the same time forcing the wire to be thinner in order to fit into the stator slots. The distribution factor necessitated by the use of three phase power lowers the efficiency of the machine by increasing resistance losses in the windings. High phase order machines may be wound with concentrated windings, eliminating these problems.
2) High phase order machines tolerate drive harmonics. In a conventional three phase machine, harmonics in the drive waveform excite rotating fields which rotate at different speeds than the fundamental rotating field. These rotating fields are a source of inefficiency. This sensitivity to harmonics is especially important when inverter drives are used, because of the generally high harmonic content of the inverter output. Concentrated winding HPO machines harness the odd harmonic currents up to twice the number of phases, thus substantially reducing the losses induced by harmonic inputs. Less expensive inverter technology may be used to drive HPO systems.
3) High phase order machines tolerate air gap harmonics. In a conventional three phase machine, air gap harmonics can lead to torque ripple and reduced starting torque, and in pathological situation can cause a machine to lock into a low speed operational mode. In order to avoid air gap harmonics, three phase machines must be wound with chorded windings, that is windings which do not span 180 electrical degrees. This chording introduces a similar loss to the distribution factor, and thus reduces the efficiency of the machine in order to improve functionality with three phase power. HPO machines harness air gap harmonics, and thus may use full span windings, reducing this loss. HPO machines may also be operated at high saturation levels without difficulties being induced by saturation air gap harmonics, thus increasing overload capabilities.
4) High phase order inverter electronics makes use of a large number of comparatively small active devices. Such devices are often less expensive per unit power handling capability. Harmonic tolerance means that lower pulse rates may be used, and thus slower active devices, again a cost savings. Finally, the large number of active devices results in enhanced reliability.
Thus, for situations where long distance power transmission is not a factor, high phase order systems will be substantially more capable. Specifically, inverter driven motors wherein three phase power is converted to suitable high phase order power are a very attractive replacement for current devices.
Background--Diesel Electric Drive
A common device in the art is the diesel electric drive system. This device consists of a heat engine, generally a diesel internal combustion engine, a generator, a controller, and a motor. Mechanical power produced by the combustion of fuel is converted to electricity, which then goes to power the motor. The output of the motor is then coupled to wheels or other drive means.
This is an indirect process as compared to the direct coupling of mechanical power to the wheels. The benefits of this indirect process lies in the fact that commonly used heat engines do not function effectively at zero speed. In accelerating large inertial loads such as trains, one finds large torque requirements during low speed operation. Mechanical coupling via a transmission and clutch or torque converter would be heavy, expensive, inefficient, and subject to extreme wear. Thus indirect coupling via a generator and a controller is used.
Often the above system makes use of DC drive motors, although AC systems are entering production and use.
The benefits of these diesel electric drives are well known in the art. For other traction drive applications, still further benefits are known. For example, in mining trucks an independent motor can be placed at each wheel, providing four wheel drive with enhanced control of each wheel, a substantial improvement over the use of mechanical differentials.